1. Field of the Invention
The invention relates generally to a semiconductor wafer platform, or chuck, used to support, and by means of applied vacuum, hold, a wafer for testing in a prober station. More specifically, the invention relates to a temperature-controlled semiconductor wafer chuck which directly heats or cools the semiconductor wafer for, and during, manufacture, testing, characterization, and failure analysis of semiconductor wafers and other components at hot and cold temperatures.
2. Description of the Related Art
In the conventional manufacture of semiconductor devices, semiconductor wafers are first produced. Each semiconductor wafer can contain many individual electronic devices or electronic circuits, which are known as dies. Each die is electrically tested by connecting it to suitable test equipment. Probe pins, which are connected to the test equipment, are brought into contact with the die to be tested. This generally occurs at a prober station, which conventionally includes a prober stage supporting a wafer chuck, which in turn supports the wafer. The prober station in production applications is usually an automated testing apparatus, and such stations are well known. Alternatively the prober station can be such as will facilitate manual testing by a human operator; and in the latter case also includes a means for magnifying the wafer for observation by testing personnel, which can be employed in confirming accurate probe placement. In either case a conventional prober station also includes means for micropositioning of the prober stage, and accordingly the chuck and a wafer supported thereon, with respect to test probes etc.
It is often required to control the temperature of the die during testing, and for this purpose the semiconductor wafer chuck can be a temperature-controlled chuck. In many cases such chucks are required to be able to both heat and cool the wafer. Many types of temperature-controlled chucks are known and are widely commercially available. The simplest form consists of a chuck incorporating a heater element; and the heater heats the chuck. This design relies on natural convection to cool the chuck. This method of cooling can be too slow for many commercial production test requirements.
Temperature-controlled chucks that incorporate heaters and heat sinks are also available. Heaters can take several forms, such as plate heaters, coil heaters, mica heaters, thin film heaters, peltier elements or heater rods incorporated into the chuck. Another method involves casting the heater rods into an element of the chuck structure. Cooling is provided by a heat sink that is cooled by a recirculating fluid, or in other designs by passing a fluid through the chuck without recirculating it. The fluid can be a liquid or a gas, usually air in the latter case. The liquid or air can be chilled for greater cooling effect in passing through the chuck, and can be recirculated for greater efficiency. A chuck cooled by means of a fluid chilled to a temperature below ambient temperature enables wafer probing at temperatures below ambient. In general, current conventional heat-sink designs incorporate simple cooling channels cross-drilled and capped in the chuck base.
In order to increase their performance, the density and complexity of semiconductor devices is increasing. Feature sizes, i.e. line widths, pad areas etc. are becoming increasingly smaller. This has led to certain new requirements for temperature-controlled chucks.
As an example, the number of probe pins to be connected to each individual die is increasing. Each of these probe pins can exert a pressure of up to several ounces on each of the test pads. With a high number of probe pins, which can be several hundred over a 1 square inch area or less, the supporting chuck is put under a relatively high load due to the applied force. This force can cause the chuck to deform. Bending and displacement of the chuck, even by relatively minute amounts, can, in turn, result in the test probe pins moving off the test pads.
A solid chuck, for example, not one that is temperature controlled, can provide the structural strength required for this probing. However, insufficient rigidity can be a problem for current temperature-controlled chuck designs. Conventionally, incorporating heating and cooling means reduces the structural strength of the chuck. Current temperature-controlled chuck design is limited to less than optimal performance in providing both the thermal performance and mechanical stability required for these high probe force applications. What is needed is a temperature-controlled chuck that has good thermal performance and also a high degree of structural strength in resisting loads induced by probe pins.
For accurate positioning of the probe pins on each die, the wafer chuck must remain mechanically stable. In addition to deformation resulting from applied load forces, heating and cooling of the temperature-controlled chuck can lead to changes in the height, leveling and planarity of the top surface of the chuck, which can in turn require re-calibration of the probe station, repositioning of probe pins, and re-focusing of microscopes. Current temperature-controlled chuck designs provide less than desirable mechanical stability over the needed temperature ranges. What is needed is a temperature-controlled chuck design that will minimize changes in height, leveling and planarity over the temperature range of the chuck during testing.
As mentioned, wafer chucks are usually mounted in probe stations that, amongst other functions, position the wafer chuck such that the semiconductor die is brought into contact with the probe pins. The chuck is mounted on a prober stage that can be moved, manually or automatically, with extreme accuracy. These prober stages in particular can be sensitive to changes in temperature. Current temperature-controlled chucks provide various means of thermal insulation between the chuck and the prober stage. Over time however, thermal energy can be conducted to or from the prober stage. In cases where the temperature deviates significantly from ambient this can cause inaccuracies of positioning or damage to the prober station or prober stage. What is needed is a system that mitigates or eliminates this and the other problems discussed above.
In a first aspect of the invention, a temperature-controlled semiconductor wafer chuck system is provided which incorporates a wafer chuck having a top surface and a bottom surface, the chuck being configured for mounting on a prober stage of a wafer probe station, the system comprising a heat sink incorporating a cooler, which further comprises a fluid conduit which distributes a coolant fluid substantially uniformly through the heat sink for removing thermal energy therefrom. The heat sink further comprises pillars disposed in the fluid conduit and configured to transfer force across the fluid conduit, whereby forces applied to the top surface of the chuck are transferred through the heat sink incorporated in the chuck to the prober stage, without substantial deformation of the chuck.
In another aspect of the invention it provides a temperature-controlled semiconductor wafer chuck system including a chuck further including a chuck top plate comprising a top surface of the chuck, a chuck bottom plate comprising a bottom surface of the chuck, a first heater disposed between the heat sink and the top surface of the chuck and a second heater disposed between the heat sink and the bottom surface of the chuck. With this configuration the temperature of the top surface of the chuck and the bottom surface of the chuck can be independently controlled. In a more detailed aspect the top and bottom surfaces can be independently maintained at different desired temperatures. This is done by independently controlling the first and second heaters as required to independently add heat to the chuck adjacent the top and bottom surfaces respectively, and removing heat through the heat sink as required by action of the cooler.
In another aspect of the invention, the heat sink further comprises a cooling conduit configured for guiding a heat absorbing fluid through the heat sink. The heat sink further comprises a multiplicity of pillars extending through the cooling conduit dividing it into a multiplicity of interconnected channels, the pillars being sized and placed to transmit force applied normal to the top surface of the chuck through the heat sink to the bottom surface of the of the chuck. In a more detailed aspect the heat sink can comprise at least two elements permanently joined to form an integral structure, at least one of the elements having the interconnected channels formed in one side and the other element closing the channels to form a fluid tight cooling conduit within a relatively rigid structure. Due to its configuration the heat sink substantially uniformly resists compressive loads due to forces applied in a direction perpendicular to the top surface of the chuck with minimum deformation, and also resists bending of the chuck when probe loads are applied nearer to the outer edges of the chuck in areas not directly supported underneath by a prober stage. The placement of the pillars can be such that it forms a uniform pattern.
In further more detailed aspects, the configuration of the cooling channel can be such that cooling fluid enters and exits the chuck at locations adjacent one another, the path of fluid flow being essentially U-shaped. The fluid channel can be formed by a machining or a casting process in fabrication of the heat sink. The majority of pillars can incorporate comers, tending to induce turbulent flow of the cooling fluid rather than laminar flow, improving efficiency.
Also in a more detailed aspect, the first or primary heater and the second or secondary heater can both be electrical resistance coil type heaters. They can employ a direct current power source.
In additional more detailed aspects, the temperature-controlled wafer chuck system can further comprise a first temperature sensor configured to monitor temperature at the top surface of the chuck, and a second temperature sensor configured to monitor the temperature adjacent the bottom surface of the chuck, the temperature sensors each generating a signal which is correlated with the temperature at the location of the sensor. Multiple sensors, forming an array, can be used to take into consideration local hot and cold spots on the top and bottom of the chuck in controlling temperature. The system can further comprise a controller configured to receive signals from the first and second temperature sensors or sensor arrays and control the first and second heaters and the cooler so as to control the temperature at the top and bottom surfaces of the chuck. A user interface can be employed, enabling selection of desired temperatures and communication of sensed temperatures at the top and bottom surfaces of the chuck. When the heaters are electrical, the controller can be programmed so that it raises or lowers the temperature at the top and bottom surfaces by increasing or decreasing the power supplied to a heaters gradually, rather than simply turning them on and off. This can be done so as to avoid electrical noise attendant sudden changes in current passing through the heater(s).
In a further more detailed aspect, the system can be configured so that the controller maintains the temperature of the bottom surface of the chuck at substantially the same temperature as the prober stage of a probing station during thermal test probing of a wafer, to minimize thermal effects on the prober stage during such testing.
In another more detailed aspect, the temperature-controlled wafer chuck of the system can further comprise an isolation layer disposed between the heater atop the heat sink and the top plate, said isolation layer being formed of an electrically non-conductive material. The temperature-controlled wafer chuck system can further include an electrically conductive guard layer disposed on top of the isolation layer, and a further isolation layer between the guard layer and the chuck top. With this configuration the guard layer and chuck top can be brought to the same electrical potential to further reduce current leakage through the chuck and further electrically isolate a wafer supported on top of the chuck during testing from electrical noise generated by heaters and prober mechanisms.
In another aspect of the invention the temperature-controlled wafer chuck can further comprise a differential expansion and contraction accommodation assembly, whereby distortion of the chuck due to differences in expansion and contraction between layer elements such as the top plate and the isolation layer and the heat sink and heater elements is minimized. The expansion and contraction accommodation assembly further can comprise over-sized and/or slotted holes in the elements and bolts and washers allowing relative movement between adjacent elements and minimizing distortion of the elements.
In the case of a chuck configured to also incorporate a conductive guard layer and further isolation layer, facilitating trivial or guarded testing of semiconductor wafer dies, increased potential for distortion exists and such a chuck can also further comprise a differential expansion and contraction accommodation assembly incorporated in a mechanical connection between the additional layer elements, whereby distortion of the chuck due to differential expansion and contraction of the layer elements is minimized.
Additionally, a chuck configured in accordance with principles of the invention can also be configured so that a vacuum can be created between the layer elements to further hold them in place in close proximity to each other while allowing relative movement due to differential thermal expansion.
As will be appreciated, a temperature-controlled chuck made in accordance with principles of the invention has good thermal performance, and also a high degree of structural strength in resisting loads induced by prober pins. The invention incorporates new design and assembly features that help minimize changes in height, flatness and planarity over the temperature range of the chuck. The chuck incorporates a method and apparatus for actively controlling the temperature of the base of the chuck to eliminate the problem of thermal effects on the prober stage, for example by matching the temperature of the bottom of the chuck to the temperature of the prober stage, which usually is the same as an ambient environmental temperature.
There are other objective to the present invention. For example, one such objective is to lower the thermal resistance of the chuck. Yet another objective is to minimize the number of parts in manufacturing the chuck, to lower the cost in producing the chuck. Still another objective of is to minimize electrical noise on the chuck, without grounding the chuck.